[Qemu-devel] Re: [PATCH v2 14/17] kvm: x86: Introduce kvmclock device to save/restore its state

[Jan Kiszka <jan.kiszka@web.de>]

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Am 03.01.2011 17:04, Avi Kivity wrote:
> On 01/03/2011 10:33 AM, Jan Kiszka wrote:
>> From: Jan Kiszka<jan.kiszka@siemens.com>
>>
>> If kvmclock is used, which implies the kernel supports it, register a
>> kvmclock device with the sysbus. Its main purpose is to save and restore
>> the kernel state on migration, but this will also allow to visualize it
>> one day.
>>
> 
> kvmclock is a per-cpu affair.

Nope, it's state (the one save/restored here) is per VM.

> 
>>
>> @@ -534,6 +599,10 @@ int kvm_arch_init(int smp_cpus)
>>       int ret;
>>       struct utsname utsname;
>>
>> +#ifdef KVM_CAP_ADJUST_CLOCK
>> +    sysbus_register_withprop(&kvmclock_info);
>> +#endif
>> +
> 
> So this doesn't look right.  I think we're fine with just migrating the
> MSRs, like we migrate anything else that has to do with the cpu.
> 

The kvmclock state is not contained in any MSR. It's an independent
machine state that can be indirectly obtained via MSR access. Therefore,
qemu-kvm currently registers only one vmstate entry per machine, and
this patch just turns this into a clean device - because that's what
kvmclock is in the end, something like an HPET.

Jan


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